1. Field of the Invention
The present invention generally relates to semiconductor integrated circuit chips and packages therefor and, more particularly, to interlayer interconnects and fine pitch intralevel interconnects in the connection pad layer in semiconductor integrated circuits and packages therefor.
2. Description of the Prior Art
Extremely high integration density has been pursued in recent years due to the increases in performance due to reduced signal propagation time as well as increased functionality and increased economy of manufacture. Nevertheless, integrated circuit complexity is primarily limited by the ability to make a sufficient number of power and signal connections to and from a given chip to a package and vice-versa. The same limitation applies to the interface of the package and the board or device in which the package is included. Since a chip of sufficient complexity to require only power, data and control inputs and outputs (e.g. a system-on-a-chip) is not generally available at the present time (except as relatively simple application specific integrated circuits (ASICs)), many hundreds if not thousands of connections between a chip and its packaging are generally required. Numerous wire bonding and solder preform techniques (e.g. C4) are well-known for making such numerous and closely spaced connections with high reliability.
However, to increase signal propagation speed and enhance the effects of extremely high integration density, the use of copper wiring interconnects on the chip and in packaging has become the metallurgy of choice for high performance devices. At the same time, aluminum metallurgy is preferred for final interconnect and so-called distribution layer interconnections due to the ability of forming extremely fine interconnects at fine pitch and in close proximity with each other where the interconnects can be encapsulated to eliminate significant metal migration. Further, aluminum metallurgy is preferred for chip and package solder and wire bonding connection pads. Wire bond and C4 solder connections are considered to require a last metal connection pad layer of aluminum due to, at least, the solubility of copper in solder materials and other metals causing the development of undesired structures or damaging effects as copper precipitates upon cooling.
To integrate aluminum wiring and pad metal levels with lower copper wiring metal layers, it has been necessary to form large copper plugs or pads overlaid by a large aluminum via and a large aluminum pad to make connections to aluminum layers. However, such a large pad effectively prevents interconnection wiring from being provided in the same aluminum layer, at least in the numbers and complexity of interconnections generally required for many integrated circuits at the present time.
For connection between aluminum wiring and/or pad levels, vias much smaller than the copper and aluminum pad and via interface structures can be used. Tungsten has become a metal of choice for such vias or studs between aluminum metallurgy layers and suitable tungsten via/stud formation processes are well-known. Tungsten studs have generally been used for their low resistivity, good reliability and good adhesion to aluminum. However, tungsten fluoride (WF6) must be used for chemical vapor deposition processes to deposit the tungsten studs in vias of the chip or package.
Unfortunately, tungsten fluoride tends to attack copper causing corrosion and some potentially significant and uncontrolled etching of the copper. This effect can unpredictably cause reduction in manufacturing yield and/or variation of device electrical properties. Certainly, processes which allow for uncontrolled chemical processes to take place will not support foreseeable increases in integration density and smaller sizes and finer pitches of copper connections. For this and other possible reasons, tungsten studs have not been used to connect to copper
It should be understood that the formation of a metal layer is a time-consuming process; adversely affecting tool throughput and productivity. Further, there is a large economic cost associated with the duration of such processes and tool and facility amortization represents a substantial fraction of device cost. These problems are aggravated by the fact that copper processes, per level, are more time-consuming than aluminum layer processes for suitable interconnects made below the copper to aluminum interface structure layer. Moreover, it should be recognized that the copper to aluminum interface structures require essentially the same pattern to be formed in two separate metal layers which, together, have the sole function of altering metallurgy to accommodate solder and wire bonding connections at yet another metal layer level.
This cost of copper processes may be well-justified at lower interconnect layers where signals are being communicated between active devices on the chip and where critical paths are much more likely to exist for correct functioning of the overall integrated circuit. However, once the full functionality of the chip has been achieved, leading signals on and off the chip is much less critical and the additional delay is generally small relative to the overall path length to be traversed between chips. By the same token, the processing time required for provision of a copper wiring layer (for optimum device performance), together with an aluminum layer represents a significant portion of the total wafer processing time for a state of the art, high density integrated circuit chip or wafer. Therefore, the costs per chip of known structures for integration of copper and aluminum may be substantial for each chip but have been unavoidable when performance requirements have left no alternative to integration of copper and aluminum.